Embodiments of the present invention generally relate to photovoltaic cells and methods for making photovoltaic cells. Specific embodiments pertain to photovoltaic cells and methods of making photovoltaic cells having a substantially minimized dead zone.
Thin-film photovoltaic devices are typically fabricated as monolithic integrated modules where multiple solar cells are monolithically interconnected by way of a series of patterns and depositions. These patterns are typically effected by laser or mechanical scribing using expensive, high precision patterning tools that add cost and complexity to the photovoltaic device manufacturing.
A typical manufacturing process for solar cells is shown in FIG. 1. Starting at 100, solar cells are manufactured by starting with a glass sheet or substrate 114. An exemplary thickness for the glass sheet is about 3 mm. In the art, this glass substrate is typically called a glass superstrate because sunlight will enter through this support glass. During the manufacture of a solar cell, shown in step 102, a continuous, uniform front contact layer 116, typically including a transparent conductive oxide (TCO) is deposited on the glass substrate 114. The thickness of the front contact layer 116 is typically a few hundred nanometers. The front contact layer 116 eventually forms the front electrodes of the solar cell. Suitable materials for the front contact layer 116 include, but are not limited to, aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), indium molybdenum oxide (IMO), indium zinc oxide (IZO) and tantalum oxide. The front contact layer 116 can be deposited by any suitable process, such as chemical vapor deposition (CVD).
In step 104, after the deposition of the front contact layer 116, a laser scribing process, which is often referred to as P1, scribes strips 118 through the entire thickness of the front contact layer 116. The scribed strips are usually 5-10 mm apart. After the scribing process P1, a p- and n-type silicon layer 120 is deposited over the front contact layer 116, as shown in step 106. The total thickness of the silicon layer 120 is typically on the order of 2-3 μm, and this layer is usually deposited by chemical vapor deposition or other suitable processes.
Referring to step 108, the silicon deposition step is followed by a second laser scribing step, often referred to as P2, which completely cuts strips 122 through the silicon layer 120. As shown in step 110, a back contact layer 124, which often includes one or more of a TCO layer and a metal layer that forms the rear electrode is deposited over the silicon layer 120. The back contact layer 124 can be deposited by any suitable deposition process, such as physical vapor deposition (PVD). Referring now to step 112, a third scribing process, called P3, is used to scribe strips 126 through the back contact layer 124 and the silicon layer 120. The panel is then typically sealed with a rear surface glass lamination (not shown). The area between, and including, the P1 and P3 scribes results in a dead zone 128 which decreases the overall efficiency of the cell. The dead zone is typically in the range of about 100 μm to about 500 μm, depending on the accuracy of the lasers and optics employed in the scribing processes.
Therefore, there is a need to provide methods to improve the efficiency and ease of manufacturing photovoltaic cells.